50 Common Reverse Osmosis Questions and Solutions: Solve 99% of Your Problems
This article consists of two sections.
The first covers 26 common issues with reverse osmosis systems;
The second covers 24 common issues with reverse osmosis equipment;
Estimated reading time: 25 minutes. We recommend skimming through the content as needed.
Part 1 Common Issues with Reverse Osmosis Systems
1. How often should a reverse osmosis system be cleaned?
Generally, the RO system should be cleaned when the standardized flux decreases by 10–15%, the system’s desalination rate decreases by 10–15%, or the operating pressure and inter-stage pressure drop increase by 10–15%. Cleaning frequency is directly related to the level of system pretreatment. When SDI15 < 3, cleaning may be required four times a year; when SDI15 is around 5, the frequency may need to be doubled, though the actual frequency depends on the specific conditions at each project site.
2. What is SDI?
Currently, the most effective method for evaluating the potential for colloidal contamination in the feedwater of RO/NF systems is to measure the Sediment Density Index (SDI, also known as the fouling index). This is a critical parameter that must be determined prior to RO system design. During RO/NF operation, measurements must be taken regularly (2–3 times daily for surface water). ASTM D4189-82 specifies the standard for this test. The feedwater requirement for membrane systems is that the SDI15 value must be ≤5. Effective pretreatment technologies for reducing SDI include multimedia filters, ultrafiltration, and microfiltration. Adding polyelectrolytes prior to filtration can sometimes enhance the effectiveness of these physical filtration methods and further reduce the SDI value.
3. Should reverse osmosis or ion exchange be selected for typical feedwater?
Under many feedwater conditions, both ion exchange resin and reverse osmosis are technically feasible; the choice of process should be determined by a cost-benefit analysis. Generally, the higher the salinity, the more economical reverse osmosis becomes; conversely, the lower the salinity, the more economical ion exchange becomes. Due to the widespread adoption of reverse osmosis technology, combined processes such as reverse osmosis + ion exchange, multi-stage reverse osmosis, or reverse osmosis combined with other advanced desalination technologies have become recognized as technically and economically sound water treatment solutions. For further details, please consult a representative from a water treatment engineering company.
4. How many years do reverse osmosis membrane elements typically last?
The service life of a membrane depends on its chemical stability, the physical stability of the element, cleanability, feedwater source, pretreatment, cleaning frequency, and operational management. Based on economic analysis, it is typically 5 years or more.
5. What is the difference between reverse osmosis and nanofiltration?
Nanofiltration is a membrane-based liquid separation technology positioned between reverse osmosis and ultrafiltration. Reverse osmosis can remove the smallest solutes with molecular weights less than 0.0001 microns, while nanofiltration can remove solutes with molecular weights around 0.001 microns. Essentially, nanofiltration is a form of low-pressure reverse osmosis used in applications where the purity of the treated water is not particularly stringent. It is suitable for treating well water and surface water. Nanofiltration is appropriate for water treatment systems that do not require the high desalination rates of reverse osmosis, yet it exhibits a high capacity for removing hardness components and is sometimes referred to as a “softening membrane.” Nanofiltration systems operate at lower pressures and consume less energy than comparable reverse osmosis systems.
6. What separation capabilities does membrane technology offer?
Reverse osmosis is currently the most precise liquid filtration technology. Reverse osmosis membranes retain inorganic molecules such as soluble salts and organic compounds with molecular weights greater than 100, while allowing water molecules to pass freely through. The typical removal rate for soluble salts is >95–99%. Operating pressures range from 7 bar (100 psi) for brackish water feed to 69 bar (1,000 psi) for seawater. Nanofiltration can remove impurities as small as 1 nm (10 Å) and organic compounds with molecular weights greater than 200–400. The removal rate for dissolved solids is 20–98%. The removal rate for salts containing monovalent anions (such as NaCl or CaCl₂) is 20–80%, while salts containing divalent anions (such as MgSO₄) have a higher removal rate of 90–98%.
Ultrafiltration separates macromolecules larger than 100–1,000 Å (0.01–0.1 μm). All dissolved salts and small molecules can pass through the ultrafiltration membrane, while substances such as colloids, proteins, microorganisms, and large organic molecules are removed. The molecular weight cutoff of most ultrafiltration membranes ranges from 1,000 to 100,000. Microfiltration removes particles in the range of approximately 0.1–1 micrometers. Typically, suspended solids and large colloidal particles are retained, while macromolecules and dissolved salts pass freely through the microfiltration membrane. Microfiltration membranes are used to remove bacteria, micro-flocs, or total suspended solids (TSS), with a typical pressure difference across the membrane of 1–3 bar.
7. Who sells membrane cleaning agents or provides cleaning services?
Water treatment companies can provide specialized membrane cleaning agents and cleaning services. Users may also purchase cleaning agents on their own based on recommendations from the membrane manufacturer or equipment supplier to perform membrane cleaning.
8. What is the maximum allowable silica concentration in the feedwater for reverse osmosis membranes?
The maximum allowable silica concentration depends on temperature, pH, and scale inhibitors. Typically, without scale inhibitors, the maximum allowable concentration in the concentrate stream is 100 ppm. Certain scale inhibitors may allow silica concentrations in the concentrate stream as high as 240 ppm; please consult your scale inhibitor supplier.
9. What effect does chromium have on RO membranes?
Certain heavy metals, such as chromium, can catalyze the oxidation of chlorine, leading to irreversible performance degradation of the membranes. This is because Cr⁶⁺ is less stable in water than Cr³⁺. It appears that metal ions with higher oxidation states exhibit a stronger destructive effect. Therefore, the concentration of chromium should be reduced during pretreatment, or at least Cr⁶⁺ should be reduced to Cr³⁺.
10. What kind of pretreatment is generally required for an RO system?
A typical pretreatment system consists of the following: coarse filtration (~80 microns) to remove large particles, addition of oxidizing agents such as sodium hypochlorite, followed by fine filtration via a multimedia filter or clarification tank, then addition of sodium bisulfite to reduce residual chlorine and other oxidizing agents, and finally installation of a security filter before the high-pressure pump inlet. As the name implies, the security filter serves as a final safeguard to prevent accidental damage to the high-pressure pump impeller and membrane elements caused by large particles. Water sources with high levels of particulate matter typically require a higher degree of pretreatment to meet specified feedwater requirements; for water sources with high hardness, softening, acid addition, or scale inhibitors are recommended; and for water sources with high levels of microorganisms and organic matter, activated carbon or anti-fouling membrane elements are also required.
11. Can reverse osmosis remove microorganisms such as viruses and bacteria?
Reverse osmosis (RO) is highly effective, achieving very high removal rates for viruses, bacteriophages, and bacteria—at least 3log or higher (removal rate >99.9%). However, it is important to note that in many cases, microbial regrowth may still occur on the permeate side of the membrane. This primarily depends on the methods of installation, monitoring, and maintenance. In other words, a system’s ability to remove microorganisms hinges on whether the system design, operation, and management are appropriate, rather than on the properties of the membrane elements themselves.
12. How does temperature affect water production?
Higher temperatures result in higher water production, and vice versa. When operating at higher temperatures, the operating pressure should be reduced to maintain constant water production, and vice versa. Please refer to the relevant section for the temperature correction factor (TCF) regarding changes in water production.
13. What are particulate and colloidal fouling?
How are they measured? Once particulate and colloidal fouling occurs in a reverse osmosis or nanofiltration system, it severely affects membrane water production and may also reduce the desalination rate. An early symptom of colloidal fouling is an increase in system pressure drop. The sources of particles or colloids in the membrane feedwater vary by location but often include bacteria, sludge, colloidal silica, and iron corrosion products. Chemicals used in the pretreatment stage, such as polyaluminum chloride, ferric chloride, or cationic polyelectrolytes, may also cause fouling if they are not effectively removed in clarification tanks or media filters. Furthermore, cationic polyelectrolytes may react with anionic scale inhibitors, and the resulting precipitates can foul the membrane elements. The tendency for such fouling in the water or the adequacy of pretreatment can be evaluated using the SDI15 index; please refer to the detailed description in the relevant section.
14. What is the maximum allowable downtime without system flushing?
If the system uses scale inhibitors, the allowable downtime is approximately 4 hours when the water temperature is between 20°C and 38°C; approximately 8 hours when the temperature is below 20°C; and approximately 1 day if the system does not use scale inhibitors.
15. Can a reverse osmosis (RO) pure water system be frequently started and stopped?
Membrane systems are designed for continuous operation; however, in actual operation, there will inevitably be a certain frequency of startup and shutdown. When the membrane system is shut down, it must be flushed at low pressure using its own product water or water that has passed pretreatment to displace the high-concentration brine (which contains scale inhibitors) from the membrane elements. Measures should also be taken to prevent water leakage and air ingress into the system, as dehydration of the elements may result in irreversible loss of permeate flux. If the shutdown duration is less than 24 hours, no measures to prevent microbial growth are required. However, if the shutdown exceeds this duration, the system should be preserved using a protective solution or the membrane system should be flushed at regular intervals.
16. How is the orientation of the brine seal ring on the membrane element determined?
The brine seal ring on the membrane element must be installed at the feed end of the element, with the opening facing the feed direction. When water is fed into the pressure vessel, the opening (lip) will expand further, completely sealing off any bypass flow between the membrane element and the inner wall of the pressure vessel.
17. How is silica removed from water?
Silica in water exists in two forms: active silica (monomeric silica) and colloidal silica (polymeric silica): Colloidal silica lacks ionic characteristics but has relatively large particle sizes. It can be retained by fine physical filtration processes, such as reverse osmosis, and its concentration in water can also be reduced through coagulation techniques, such as coagulation clarification tanks. However, separation technologies that rely on ionic charge characteristics, such as ion exchange resins and continuous deionization (CDI) processes, are very limited in their ability to remove colloidal silica. Active silica is much smaller in size than colloidal silica, so most physical filtration techniques—such as coagulation and clarification, filtration, and dissolved air flotation—cannot remove active silica. Processes capable of effectively removing active silica include reverse osmosis, ion exchange, and continuous electrodialysis.
18. How does pH affect removal efficiency, water production, and membrane lifespan?
Reverse osmosis membranes typically operate within a pH range of 2 to 11. pH has a minimal direct impact on membrane performance itself—a key distinction from other membrane products. However, the properties of many ions in water are significantly affected by pH. For example, weak acids like citric acid primarily exist in a non-ionic state at low pH, whereas they dissociate into ions at high pH. Since the same ion exhibits a higher removal rate by the membrane when it is highly charged, and a lower removal rate when it is less charged or uncharged, pH has a significant impact on the removal rate of certain impurities.
19. What is the relationship between feedwater TDS and conductivity?
When feedwater conductivity values are obtained, they must be converted to TDS values for input into software design. For most water sources, the conductivity-to-TDS ratio ranges from 1.2 to 1.7. For ROSA design, a conversion ratio of 1.4 is used for seawater and 1.3 for brackish water, which typically yields a good approximation.
20. How can you tell if the membrane has become fouled?
The following are common symptoms of fouling:
- A decrease in water production at standard pressure;
- The need to increase operating pressure to achieve standard water production;
- An increase in the pressure drop between the feed and concentrate streams;
- An increase in the weight of the membrane elements;
- A significant change in membrane rejection rate (increase or decrease).
- When an element is removed from the pressure vessel, if water is poured onto the feed side of the upright membrane element and the water cannot flow through the element but only overflows from the end face (indicating that the feed water channel is completely blocked)
21. How can microbial growth be prevented in the original packaging of membrane elements?
If the preservation solution appears cloudy, it is likely due to microbial growth. Membrane elements preserved with sodium bisulfite should be inspected every three months. If the preservation solution becomes cloudy, remove the elements from the sealed storage bag and re-immerse them in fresh preservation solution. The solution should be a 1% (by weight) food-grade sodium bisulfite solution (not cobalt-activated). Soak for approximately one hour, then reseal and store. Drain the elements thoroughly before repackaging.
22. What are the feedwater requirements for RO membrane elements and IX ion exchange resins?
Theoretically, the feedwater entering RO and IX systems should be free of the following impurities: suspended solids, colloids, calcium sulfate, algae, bacteria, oxidizing agents (such as residual chlorine), oils or fats (which must be below the instrument’s detection limit), organic matter and iron-organic complexes, as well as metal oxides such as iron, copper, and aluminum corrosion products. Feedwater quality has a significant impact on the service life and performance of RO elements and IX resins.
23. What impurities can RO membranes remove?
RO membranes are highly effective at removing ions and organic compounds. Reverse osmosis membranes have a higher removal rate than nanofiltration membranes. Reverse osmosis typically removes 99% of the salts in the feedwater, and the removal rate for organic compounds in the feedwater is ≥99%.
24. How do you determine which cleaning method to use for your membrane system?
To achieve the best cleaning results, it is crucial to select cleaning agents and procedures that address the specific issue. Incorrect cleaning can actually worsen system performance. Generally, acidic cleaning solutions are recommended for inorganic scaling contaminants, while alkaline cleaning solutions are recommended for microbial or organic contaminants.
25. Why is the pH of RO product water lower than that of the feed water?
In a closed system, the relative concentrations of CO₂, HCO₃⁻, and CO₃²⁻ vary with pH. At low pH, CO₂ predominates; in the moderate pH range, HCO₃⁻ is the primary component; and at high pH, CO₃²⁻ is the main component. Since RO membranes can remove dissolved ions but not dissolved gases, the CO₂ content in RO permeate is essentially the same as that in the RO feedwater; however, HCO₃⁻ and CO₃²⁻ are often reduced by 1 to 2 orders of magnitude. This disrupts the equilibrium between CO₂, HCO₃⁻, and CO₃²⁻ in the feedwater. In a series of reactions, CO₂ combines with H₂O, causing a shift in the following reaction equilibrium until a new equilibrium is established.
CO₂ + H₂O → HCO₃⁻ + H⁺
If the feedwater contains CO₂, the pH of the RO membrane product water will always decrease. For most RO systems, the pH of the product water will drop by 1 to 2 pH units. When the feedwater alkalinity and HCO₃⁻ concentration are high, the decrease in product water pH is even greater. In very few cases, where the feedwater contains low levels of CO₂, HCO₃⁻, or CO₃²⁻, the change in product water pH is minimal. If the pH of the RO permeate is too low, a metering pump is used to add NaOH to adjust the pH to an alkaline level, as the desalination efficiency of reverse osmosis is optimal when the pH is between 7.5 and 8.
26. How can the energy consumption of a membrane system be reduced?
Low-energy-consumption membrane elements can be used, but it should be noted that their desalination rates are slightly lower than those of standard membrane elements.
Part 2: Summary of Common Issues with Reverse Osmosis Systems
1. Why do O-rings in reverse osmosis systems swell?
In water treatment systems, reverse osmosis units require three types of O-rings to ensure the sealing and isolation of different compartments within the membrane housing. To reduce installation resistance, clean water or glycerin should be applied to the surface of each seal during system installation. It is important to note that petroleum-based lubricants such as Vaseline should be used with caution, as they can cause cracking in the freshwater pipes and, more importantly, lead to the swelling of the seals. While seal swelling generally does not directly affect system performance, it can complicate reinstallation after system disassembly, as the swollen seals may be difficult to reinsert into their slots.
2. Is the water production rate consistent across all components in the reverse osmosis system?
In a reverse osmosis system, a pressure difference—known as the membrane pressure drop—exists between the feed water side and the concentrate side of each membrane element. Since the salt concentration in the concentrate of each element is higher than that in the feed water, the osmotic pressure of the feed water increases progressively along the system flow path. If we ignore the backpressure and osmotic pressure of the permeate, the water production of each membrane element along the system flow will be directly proportional to the difference between its operating pressure and the osmotic pressure, meaning the water production of each membrane element gradually decreases.
3. Does pH affect the removal efficiency and lifespan of reverse osmosis membranes?
As reverse osmosis (RO) systems represent a primary filtration process in water treatment, the pH of the feedwater entering the RO membranes is a concern regarding potential damage. Generally, RO membranes are made of composite membrane materials. When used within the specified pH range—typically 2 to 11—the pH has minimal impact on the membrane itself. As for the impact of pH on the desalination rate of reverse osmosis membranes, this is determined by the characteristics of various ions in the water, which are influenced by the pH value. This is due to the ions’ inherent acidity or alkalinity, their decomposition properties, and their charge levels—all of which can lead to a reduction in the membrane’s desalination rate. It is evident that pH has a significant impact on the desalination rate of certain impurities. Similarly, if the reverse osmosis membrane has zero CO₂ removal efficiency, increasing the pH of the feedwater to convert CO₂ into CO₃²⁻ allows the membrane to effectively remove salts. However, special attention must be paid to the risk of scaling on the reverse osmosis membrane in this scenario.
4. How should reverse osmosis equipment be operated during initial startup?
Use a low-pressure, low-flow approach to purge air from the piping; the system can only operate normally once all air has been removed. First, maintain the pressure between 0.2 and 0.4 MPa. When flushing and venting at low pressure, discharge the resulting concentrate and permeate water into the sewer system. If the pressure rises rapidly during operation, this indicates the presence of air within the membrane elements, which will generate radial impact forces from the water flow. Under such conditions, the membrane’s outer casing may rupture, resulting in irreparable damage to the reverse osmosis membrane. During initial use, the operating pressure must be adjusted to 0.2–0.4 MPa for flushing, and ensure that the reverse osmosis system automatically performs a low-pressure flush of the membranes each time it is started.
5. How to Replace the Security Filter Cartridge in a Reverse Osmosis System?
After a period of use, the water quality treated by the security filter may vary due to prolonged operation. This can lead to clogging of the filter cartridge. Replacement should be based on the pressure differential across the filter; when the differential exceeds 0.03 MPa, replacement should be considered. Replacement procedure: Shut down the reverse osmosis system. Relieve the pressure by pressing the pressure relief valve button on the equipment until the pressure gauge reads zero. Use a specialized wrench to unscrew the filter housing. Remove the old filter cartridge and install the new one. Use the specialized wrench to screw the filter housing back on tightly.
6. How to Clean and Disinfect a Reverse Osmosis System?
Generally, cleaning of a reverse osmosis system should be performed by professional technicians; customers should not attempt to clean it themselves. If the system requires cleaning, contact a professional service provider to complete the task. The following conditions indicate the need for chemical cleaning; if any of these occur, cleaning should be performed. Prerequisites for chemical cleaning: When the system’s water production rate decreases by 5–10% compared to initial operation or the previous cleaning. When the system’s desalination rate decreases by 2.5–5% compared to initial operation or the previous cleaning. When the pressure differential across system stages is 1–2 times higher than during initial operation or after the previous cleaning. When the system is to be shut down for an extended period, it should be protected with a preservation solution. Note: Water production and desalination rates are affected by water temperature; therefore, test results should be obtained under consistent water temperature conditions.
7. Is the fluoride removal efficiency of reverse osmosis systems effective?
Consuming water with excessive fluoride levels can be harmful to human health. To protect people from the hazards of fluoride in daily life, reverse osmosis systems can be used for fluoride removal. Most fluoride ions in groundwater originate from the dissolution of surrounding rock due to erosion. Since water also contains a large amount of soluble ions, the impact of fluoride removal on other molecules must be considered during the process. In groundwater with excessively high salinity, the fluoride removal rate achieved by reverse osmosis equipment is not particularly high. However, compared to other methods, reverse osmosis is simple to operate and provides effective treatment results.
8. What are the basic requirements for the quality of water produced by purified water systems?
Purified water produced by purified water systems is used in industries such as pharmaceuticals and biomedicine. But what are the basic standards for purified water quality? Both the Chinese Pharmacopoeia and the European Pharmacopoeia clearly state that source water for pharmaceutical use must at least meet drinking water standards. If it does not meet these standards, it must undergo pretreatment until it does. Apart from specific regulations regarding E. coli, the total bacterial count must not exceed 100 CFU/ml. During the water production process, internal contamination may occur, and various water treatment components within the system can serve as potential sources of contamination. Therefore, it is essential to regularly clean and disinfect the system. Additionally, a sterilization and disinfection unit should be installed at the outlet of the purified water system.
9. What are the characteristics of the water quality produced by a purified water system?
The water produced by the purified water system complies with national hygiene standards and meets the enterprise’s actual production standards. The output water—purified water—exhibits two key characteristics. One characteristic is the increasing installation of disinfection and sterilization equipment within the system. The other is that the system’s distribution piping has replaced traditional supply lines with recirculating lines. Both of these features are designed to control microbial contamination and the accumulation of bacterial endotoxins. At the same time, attention must be paid to the impact of flow velocity within the pipes on microbial proliferation; that is, if the flow velocity is too low or the pipes become blocked, it may lead to increased microbial growth, thereby affecting water quality.
10. What are the key considerations for selecting the installation location of water softening equipment?
Water softening equipment produces high-quality water and operates stably, making it widely used by many enterprises. When installing water softening equipment, the following points should be noted:
1) Water softening equipment should be installed as close as possible to the drainage point.
2) If other water treatment equipment is required, space should be reserved for its installation.
3) Since salt is frequently added to the salt tank, a designated area for salt storage should be provided.
4) Do not install the water softening equipment within 3 meters of a boiler, as hot water may flow back into the system and cause damage.
5) The water softening equipment should be placed in an environment where the ambient temperature is between 1°C and 49°C.
11. What are the precautions for using water softening equipment?
Water softening equipment is designed to remove Ca, Mg, and other ions from water, reducing water hardness and softening the water. After prolonged use, certain precautions must be followed.
1) The inlet valve of the water softening equipment should remain open at all times during operation, unless the equipment is undergoing maintenance. If water is not required, the outlet valve may be closed.
2) If the output water fails to meet standards due to incomplete regeneration, manual regeneration can be performed.
3) When the water softener is idle for an extended period, brine must be drawn into the tank for protection.
4) When resuming operation, simply prepare brine as you would for normal operation. First, open the inlet valve, then connect the power supply. Allow the unit to reset automatically, perform one manual regeneration cycle, and then open the outlet valve.
5) If the industrial salt used is too dirty, the salt tank should be cleaned once a year to ensure the salt-drawing process proceeds smoothly.
12. What are the installation standards for ultrapure water systems?
Ultrapure water systems produce high-quality water suitable for the production needs of various enterprises, and their scope of application is gradually expanding. This article primarily outlines the installation standards for ultrapure water systems.
1) The installation site should be level, clean, and located near a power source and water supply.
2) Keep the equipment away from fire sources and any heat-generating objects to prevent heat from affecting its operational efficiency.
3) In northern regions, the equipment must not be installed outdoors to prevent internal freezing, which could damage instruments and filter elements.
4) The installation location should allow for convenient drainage to ensure the equipment’s drain pipes remain unobstructed.
5) Ensure the operating pressure of the system’s water pump is maintained between 1.0 and 1.2 MPa, and ensure the pump operates within its rated head range.
13. Troubleshooting for booster pumps and high-pressure pumps failing to draw water:
For 380V systems, check whether the booster pump or high-pressure pump is rotating in the wrong direction. If so, swap any two of the pump’s three power terminals. If the pump is not rotating in the wrong direction, open the pump’s air release valve to vent air or fill the pump with water. For 220V booster pumps and high-pressure pumps, reverse rotation does not occur; simply open the air release valve to vent air or fill the pump with water.
14. High-Pressure Pump Fails to Start
Solution: Check whether the relay connected to the high-pressure pump is engaged, whether the wire terminals are loose or disconnected, or if the low-water indicator light is on. If the low-water indicator light is on, it indicates that the raw water supply is insufficient for the booster pump. To prevent the high-pressure pump from running dry and causing damage, the low-water protection device cuts off the power to the high-pressure pump, thereby protecting it. If an adequate water supply is provided, allowing the low-water pressure switch to reach the high-pressure pump’s operating pressure requirement, the low-water pressure switch indicator light will turn off, and the high-pressure pump can be started.
15. The high-pressure pump emits unusual noises
Solution: Check if the high-pressure pump is running dry. Sometimes, the pump may emit unusual noises before water has fully entered the system; these noises typically subside automatically within 1–3 minutes. If the noise persists after 3 minutes, open the high-pressure pump’s air release valve to vent air or add water.
16. Pipe Burst
Solution: In some areas, poor water quality with high levels of impurities, or failure to replace the pre-filter cartridge or clean the RO membrane for an extended period, can cause the RO membrane to clog. This leads to increased pressure within the pipes, resulting in a pipe burst. If this occurs, first check whether the pre-filter cartridge needs cleaning or replacement, then clean the RO membrane. Poor-quality source water can cause frequent clogging of the RO membrane. In such cases, install an ion exchange unit or add scale inhibitors to the source water to remove impurities, thereby improving the quality of the purified water and extending the service life of the RO membrane.
17. Declining Water Output
Solution: Some users may notice that the system’s water output is decreasing (this issue rarely occurs in systems using tap water as the feed water). This is often caused by poor-quality groundwater containing high levels of impurities, which partially clog the RO membrane and reduce water output. In this case, we should regularly backwash the pretreatment system, replace the filter cartridges, clean the RO membranes, or switch from poor-quality groundwater to tap water as the feed water (if tap water is unavailable, it is advisable to install an ion exchange scale inhibition system, which will essentially prevent this issue from occurring).
18. Fine white or black suspended particles appear in the purified water
Solution: This is caused by bacterial growth resulting from contaminated pipes. To resolve this, dissolve caustic soda and place it in the precision filter. Close the wastewater valve using the control valve, open the purified water valve fully, start the high-pressure pump, and divert the water flowing from the purified water outlet into the precision filter for approximately 30 minutes of circulation. For systems equipped with a pipeline disinfection unit, promptly activate the unit to disinfect the purified water pipelines.
19. Residual gas in the reverse osmosis system
Operating under high pressure can cause water hammer, which damages the membranes. Two common scenarios include:
1) After draining the system and restarting it, pressure is rapidly increased before all gas has been fully purged. The remaining air should be purged at a pressure of 2–4 bar before gradually increasing the pressure.
2) When there is a poor seal or a leak at the connection between the pretreatment equipment and the high-pressure pump (especially leaks in the microfilter or the piping downstream of it), and when the pretreatment water supply is insufficient—such as when the microfilter becomes clogged—vacuum at the poorly sealed location may draw in air. The microfilter should be cleaned or replaced, and the piping should be ensured to be leak-free. In summary, pressure should be increased gradually only when there are no bubbles visible in the flow meter; if bubbles are observed during operation, the pressure should be gradually reduced to investigate the cause.
20. Incorrect shutdown procedures causing membrane damage
1) Rapid pressure reduction during shutdown without thorough flushing. Since the concentration of inorganic salts on the membrane concentrate side is higher than that of the feedwater, scaling is likely to occur, contaminating the membranes
. 2) Flushing with pre-treated water containing chemical additives. Water containing chemical additives may cause membrane contamination during equipment downtime. When preparing to shut down the reverse osmosis water treatment system, chemical addition should be stopped, and the pressure should be gradually reduced to approximately 3 bar. The system should then be flushed with pre-treated water for 10 minutes until the TDS of the concentrate is very close to that of the feed water.
21. Microbial contamination resulting from inadequate disinfection and maintenance of reverse osmosis equipment. This is a common issue with composite polyamide membranes, as they have poor resistance to residual chlorine. Failure to properly add chlorine or other disinfectants during operation, combined with insufficient user attention to microbial prevention, easily leads to microbial contamination. Many manufacturers’ purified water products exceed microbial limits due to inadequate disinfection and maintenance. This is primarily manifested in the following ways: the RO system was not maintained with a disinfectant solution prior to shipment; the entire piping system and pretreatment equipment were not disinfected after installation; no disinfection or maintenance measures were taken during intermittent operation; pretreatment and reverse osmosis equipment were not disinfected regularly; and the maintenance solution had expired or was insufficiently concentrated.
22. Inadequate monitoring of residual chlorine
in reverse osmosis systems—such as a malfunctioning NaHSO₃ dosing pump, expired chemical solution, or membrane damage caused by residual chlorine when activated carbon becomes saturated.
23. Damage to membrane performance due to untimely or improper cleaning.
During operation, while normal performance degradation occurs, performance decline caused by contamination is often more severe. Common types of contamination in EDI ultrapure water systems primarily include chemical scaling, organic and colloidal contamination, and microbial contamination. Different types of contamination exhibit distinct symptoms. There are also certain variations in the symptoms of membrane contamination described by different membrane manufacturers. In our engineering experience, we have found that the duration of contamination varies, and so do the symptoms. For example: When calcium carbonate fouling occurs, if the contamination period is one week, the primary symptoms are a rapid decline in desalination rate and a gradual increase in pressure drop, while the water production rate remains largely unchanged; cleaning with citric acid can fully restore performance. In a case where contamination lasted one year (in a specific water purifier), the salt flux increased from an initial 2 mg/L to 37 mg/L (with feedwater ranging from 140 mg/L to 160 mg/L), and the water production rate dropped from 230 L/h to 50 L/h. After cleaning with citric acid, the salt flux decreased to 7 mg/L, and the water production rate increased to 210 L/h. Furthermore, contamination is rarely a single-factor issue, and the symptoms it presents vary, making it more difficult to identify the specific type of contamination. To identify the type of contamination, one must comprehensively evaluate the raw water quality, design parameters, contamination indices, operational records, changes in equipment performance, and microbial indicators: 1) Colloidal contamination: When colloidal contamination occurs, it is typically accompanied by the following two characteristics: A. Rapid clogging of the microfiltration units in the pretreatment stage, especially a rapid increase in pressure drop; B. The SDI value is typically around 2.5
20. Incorrect shutdown procedures for reverse osmosis systems can damage the membranes
1) Rapid pressure reduction during shutdown without thorough flushing. Since the concentration of inorganic salts on the concentrate side of the membranes is higher than that of the feedwater, scaling is likely to occur, leading to membrane fouling.
2) Flushing with pre-treated water containing chemical additives. Water containing chemical additives may cause membrane contamination during equipment downtime. When preparing to shut down the reverse osmosis water treatment system, chemical addition should be stopped, and the pressure should be gradually reduced to approximately 3 bar. The system should then be flushed with pre-treated water for 10 minutes until the TDS of the concentrate is very close to that of the feed water.
21. Microbial Contamination Due to Inadequate Disinfection and Maintenance of Reverse Osmosis Equipment
This is a common issue with composite polyamide membranes. Because polyamide membranes have poor resistance to residual chlorine, failure to properly add chlorine or other disinfectants during operation, combined with users’ insufficient attention to microbial prevention, can easily lead to microbial contamination. Many manufacturers’ purified water products exceed microbial limits due to inadequate disinfection and maintenance. This is primarily manifested in the following ways: the RO system was not maintained with a disinfectant solution prior to shipment; the entire piping system and pretreatment equipment were not disinfected after installation; no disinfection or maintenance measures were taken during intermittent operation; pretreatment and reverse osmosis equipment were not disinfected regularly; and the maintenance solution had expired or was insufficiently concentrated.
22. Residual Chlorine in Reverse Osmosis Systems
Inadequate monitoring—such as a malfunctioning NaHSO₃ dosing pump, expired chemical solution, or saturated activated carbon—can cause membrane damage due to residual chlorine.
23. Membrane Performance Degradation Due to Untimely or Incorrect Cleaning
During operation, in addition to normal performance decline, performance degradation caused by fouling is more severe. Common types of fouling in EDI ultrapure water systems include chemical scaling, organic and colloidal contamination, and microbial fouling. Different types of fouling exhibit distinct symptoms. There are also some variations in the symptoms of membrane fouling described by different membrane manufacturers. In our engineering experience, we have found that the duration of contamination varies, and so do the symptoms. For example: when calcium carbonate fouling occurs, if the contamination period is one week, the primary symptoms are a rapid decline in desalination rate and a gradual increase in pressure drop, while the water production rate remains largely unchanged; cleaning with citric acid can fully restore performance. In a case where contamination lasted one year (in a specific pure water system), the salt flux increased from an initial 2 mg/L to 37 mg/L (with feedwater ranging from 140 mg/L to 160 mg/L), and the water production rate dropped from 230 L/h to 50 L/h. After cleaning with citric acid, the salt flux decreased to 7 mg/L, and the water production rate increased to 210 L/h. Furthermore, contamination is rarely a single-factor issue, and its symptoms vary, making identification more difficult. To determine the type of contamination, one must comprehensively evaluate the raw water quality, design parameters, contamination indices, operational records, changes in equipment performance, and microbial indicators:
1) Colloidal contamination: Colloidal contamination is typically accompanied by the following two characteristics: A. Rapid clogging of microfilters in the pretreatment stage, especially a rapid increase in pressure drop; B. An SDI value typically above 2.5.
2) Microbial contamination: In cases of microbial contamination, the total bacterial count in both the permeate and concentrate streams of the RO system is relatively high, indicating that routine maintenance and disinfection were not performed as required.
3) Scale: This can be assessed based on raw water quality and design parameters. For carbonate-type water, if the recovery rate is 75% and scale inhibitors were added during design, the LSI of the concentrate should be less than 1; if no scale inhibitors were added, the LSI of the concentrate should be less than zero, and scale generally will not form.
4) Insert a 1/4-inch PVC pipe into the module to test performance variations at different locations within the module for diagnosis.
5) Determine the type of contamination based on changes in equipment performance.
6) Use acid washing (e.g., citric acid, dilute HNO₃). Identify calcium scale based on the cleaning results and the cleaning solution, and confirm further through analysis of the cleaning solution’s composition.
7) Perform chemical analysis of the cleaning solution: Take samples of the raw water, undiluted cleaning solution, and the final cleaning solution for analysis. Once the type of contamination has been determined, clean according to the methods in Table 1, then disinfect and use. If the type of contamination cannot be determined, typically follow the cleaning (3) and disinfection steps using 0.1% HCl (pH 3).
24. Decline in Membrane Performance Due to Improper Storage and Maintenance
New reverse osmosis membrane elements are typically soaked in a 1% NaHSO₃ and 18% glycerol aqueous solution and stored in sealed plastic bags. Provided the plastic bag remains intact, storage for approximately one year will not affect the membrane’s lifespan or performance. Once the plastic bag is opened, the membranes should be used as soon as possible to prevent the oxidation of NaHSO₃ in the air, which could adversely affect the elements. Therefore, the membranes should be opened as close to the time of use as possible. After commissioning the reverse osmosis system, we have employed two methods to protect the membranes. The system is run for two days (15–24 hours), followed by preservation using a 2% formaldehyde solution; or, after running for 2–6 hours, preservation is performed using a 1% NaHSO₃ aqueous solution (air must be completely purged from the system piping, leaks must be prevented, and all inlet and outlet valves must be closed).
Both methods yield satisfactory results. The first method is more costly and is used for extended periods of inactivity, while the second method is used for shorter periods of inactivity。
